Emergency call handling in contention-based wireless local-area networks

ABSTRACT

An apparatus and methods for handling emergency message frames (e.g., “911” call frames, etc.) sent by a station in a wireless local-area network are disclosed. The illustrative embodiment increases the probability with which an emergency message frame is accorded the singularly highest quality-of-service by modifying one or more IEEE 802.11e parameters (e.g., back-off contention window length, Arbitration Inter-Frame Space [AIFS], etc.) for a station or access point that transmits an emergency message frame.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisional patentapplication Serial No. 60/444,196, filed on 3 Feb. 2003, Attorney Docket630-036us, entitled “Handling 911 Calls in a Wireless LAN,” which isincorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to telecommunications in general,and, more particularly, to techniques for handling emergency calls inwireless local-area networks.

BACKGROUND OF THE INVENTION

[0003]FIG. 1 depicts a schematic diagram of a wireless local-areanetwork 100 in the prior art, which comprises: access point 101,stations 102-1 through 102-N, wherein N is a positive integer, and hosts103-1 through 103-N, interconnected as shown. Each station 102-i,wherein i is a member of the set {1, 2, . . . N}, enables host 103-i (adevice such as a notebook computer, personal digital assistant [PDA],tablet PC, etc.) to communicate wirelessly with other hosts inlocal-area network 100 via access point 101.

[0004] QoS traffic (i.e., delay and jitter sensitive applications, likevoice and video streaming) receives special treatment on the channel ofa wireless LAN through special protocols for medium access control. Thenew draft standard for 802.11 LANs under preparation by Task Group E,referred to as 802.11e, offers a choice of two protocols, EDCF—adistributed random access protocol—and EPCF—a centralized deterministicpolling protocol.

[0005] Distributed Access Protocol: EDCF, the QoS-enhanced version ofthe existing 802.11 DCF protocol, is a distributed random accessprotocol that allows delay and jitter sensitive frames to be transmittedwith higher priority than ‘best-effort’ frames (i.e., frames that areinsensitive to delay or jitter). Frames in the higher priority accesscategories can access the channel or start countdown of their back offdelay after waiting for a shorter idle time interval following atransmission on the channel. Upon collision, priority access categorieswill double their contention window, but the maximum size achieved mayvary by access category. This allows higher priority frames to stopdoubling their contention window size sooner than lower prioritycategories, thus affording another means of differentiation.

[0006] Centralized Polling Protocol: EPCF, the point-coordinated versionof what is referred to in 802.11e as HCF, is a centralized deterministicpolling protocol that treats delay/jitter sensitive trafficpreferentially when granting opportunities for uplink transmission. In acentralized polling protocol, the Access Point (AP) sends polls to theclients granting them the opportunity to transmit. Since a nodetransmits only upon receiving a poll, transmission is contention-free.Such a protocol may waste channel time, however, if it generates pollsto stations having no data to transmit.

[0007] The choice of a protocol to use is complex, as it depends amongother things on the type of traffic generated in a WLAN cell and on theoverlap of the coverage areas of co-channel APs. While a detailedappraisal of the advantages of each protocol is outside the scope ofthis paper, it is interesting to note that in situations where themajority of the traffic in a cell is QoS traffic, the preferentialtreatment afforded to QoS frames by either protocol is irrelevant, asthere are no frames over which QoS frames would gain preferentialtreatment. The comparison must be based on delay and throughput. Thequestion then becomes: “which protocol accommodates more simultaneousindependent QoS traffic streams within acceptable delay/jitter limits”?It is easy to see that the centralized polling protocol does betterbecause channel time is not lost to contention. It is expected that, ingeneral, there will be a mix of clients in a cell, some capable ofcommunication based on the EDCF MAC protocol only, while others are ableto communicate through either MAC protocol. Some APs may not provideEPCF service.

[0008] Emergency voice calls, referred to in the U.S. as ‘911 calls’,are of special concern in wireless LANs. There are two issues thatrequire attention: call setup and voice data transmission: Attentionmust thus be paid to the following: (a) 911 calls must be set uppromptly and (b) the voice in 911 calls should be heard with clarity toenable emergency response. With respect to the former, transmission forsignaling packets must rely exclusively on the distributed MAC protocol(like EDCF), as the request to be polled (in situations where polledaccess is available) relies also on EDCF, and negotiation of thatrequest may not be completed until after call setup. With respect to thelatter, voice packets must be afforded sufficient prioritization so thatthey are transmitted within acceptable QoS specifications, regardless ofthe traffic load on the WLAN. If the 911 call is not handled with adeterministic centralized polling MAC protocol (like HCF polling),special provisions must be made for the 911 voice packets, to ensurethat they are transmitted promptly.

SUMMARY OF THE INVENTION

[0009] In an emergency 911 call, the calling client in a wireless LANwill generate signaling frames to set up a call. Signaling packets forcall setup of a 911 call must be transmitted to the AP, on the wirelesschannel, quickly and reliably. In the interest of generality, we includeamong signaling frames the TSPEC request that might be sent by theclient to reserve its position on the polling list in the case of EPCFaccess.

[0010] Signaling frames must be send through the basic channel accessmechanism, EDCF. The top priority EDCF class would be the fastest way tosend these frames. If the top priority EDCF class includes othertraffic, however, there may be substantial delay and collisions,depending on the traffic load and the ‘admission control’ policypracticed in the cell. The signaling packets would risk being dropped,due to excessive retransmission attempts.

[0011] To avoid delay or loss of 911 signaling frames, they can betreated in a special way. One way would be to transmit 911 signalingframes with the same access priority as the AP. That is, AIFS wouldbeset equal to PIFS. The backoff delay used to transmit 911 signalingframes could be set equal to 0, or to the minimum allowable backoffvalue. If a collision occurs, a station sending 911 signaling frameswould back off, just like an AP that experiences a collision whensetting up a controlled contention period (CAP); the contention windowsize for this backoff delay would be small. Alternatively, since thelikelihood of a 911 call is low, and thus collisions among 911 signalingframes are unlikely. 911 signaling frames could retransmit with the samebackoff. It is important in the latter approach to contention resolutionfor 911 frames to require the AT to access the channel with a non-zerobackoff delay following a collision.

[0012] An alternative and less aggressive approach would be to transmitthe signaling frames using ‘privileged access’. Privileged access isdescribed below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 depicts a schematic diagram of an exemplary wirelesslocal-area network 100 in the prior art.

DETAILED DESCRIPTION

[0014] Once a 911 call has been set up, voice data frames are generatedas a constant periodic stream as voice is sampled periodically. Toensure audible quality voice data frames must arrive within a specifieddelay and with limited fitter. The delay and fitter requirements can bemet more easily on the downlink (from the AP to the client) than on theuplink (from the client to the AP). A simple way for the AP to give 911frames preferential treatment on the downlink is to have 911 framesqueued separately, and to transmit them within the specified timelimits.

[0015] Uplink transmission may be problematic, depending on the MACprotocol used. Centralized deterministic polling (as in EPCF) can meetthe requirements for good voice quality more readily. The implementationof the polling schedule should give the station with the 911 callpreferential treatment; that is, it should be scheduled for polling,even if that means that another call that has been admitted and acceptedfor polling may have to be denied polling service.

[0016] Behavior of distributed random access protocols, however, is notunder the direct control of the AP. With EDCF, the voice quality QoSrequirements may not be met under heavy traffic loads. Since all clientsmay not operate under the EPCF protocol, or all APs may not offer EPCFaccess service, it is important to provide a mechanism that enables thevoice frames of a 911 call to be delivered within the specified timeconstraints.

[0017] As in the case of 911 signaling frames, voice frames of a 911call could be delivered with an AIFS of PITS. The backoff delay could beset to minimum allowable value, and collisions could be resolved byleaving the backoff unchanged. This approach, however, could beexcessively aggressive, as the probability of collision with the AP isincreased due to number of voice data frames generated at regularintervals. Frequent collisions would interfere with the smooth operationof the AP. Alternatively, privileged access can be used to transmit911-voice data frames uplink.

[0018] Privileged access would be afforded to certain frames in the toppriority access category. Privileged frames contend for the channel withthe same AIFS access parameter as the rest of the frames in the toppriority access category. However, the backoff contention window used todraw a backoff delay randomly would be different. Privileged framescould start with a shorter contention window than the frames in the toppriority access category. The probability of a collision with a framefrom another privileged client is very small, as few clients engage inapplications qualifying for privileged access. A privileged frame maycollide with non-privileged frames. In the event of a collision,retransmission of a frame may be attempted with the same, or shorter,contention window than that used previously, whereas the colliding framemust increase or retain the same contention window size upon aretransmission attempt. For better results, the contention window shouldbe shortened when attempting retransmission of a privileged frame if theframes in the top priority category maintain the same contention windowafter a collision. The probability of collision with the same frame isreduced significantly. Any subsequent collisions with other frames wouldbe resolved similarly, thus eliminating possible contenders andincreasing the probability of successful transmission of the privilegedframe.

[0019] With few clients engaged in applications that qualify forprivileged access (such as 911 calls), a shorter contention window wouldwork. A minimum size on the contention window can be imposed, ifdesired. That is, after reducing the contention window following one ormore consecutive collisions, the contention window size of a privilegedframe will remain constant.

[0020] Other frames may be afforded privileged access as well, providedthey arise infrequently and do not coincide with a 911 call. Forexample, a reservation request (RR) may be used in conjunction withvoice calls employing HCF polling. RRs would not increase contention, asthey are infrequent.

[0021] According to the draft 11e standard, a client maintains fourqueues, one for each access category. Privileged access can be achievedby using the same four queue structure, provided the client has no otherframes queued in that access category when the privileged frame reachesthe queue. The rules for setting the contention window size for the toppriority access category would be suspended for the duration of a 911call only for the station making the call, and would restored uponcompletion of the 911 call. For instance, the CWmin value could be setto 4, CWmax would be 2, with CW being halved upon collision and retrial,for the duration of a 911 call.

[0022] The mechanism of allowing the top priority category parameters tochange temporarily would not cause major unfairness in prioritizedaccess. Only the station engaged in a rare event, such as an emergencycall, would be allowed to use the privileged access parameter settings.It is unlikely that frames in the top priority category would gainpreferential treatment, as that category is reserved primarily for voicecalls. A client would typically engage in one voice call at a time.

[0023] It is to be understood that the above-described embodiments aremerely illustrative of the present invention and that many variations ofthe above-described embodiments can be devised by those skilled in theart without departing from the scope of the invention. It is thereforeintended that such variations be included within the scope of thefollowing claims and their equivalents.

What is claimed is:
 1. A method comprising: generating a first trafficstream for transmission into a shared-communications channel, whereinsaid first traffic stream is the only privileged traffic streamcontending for access to said shared-communications channel, and whereinsaid first traffic stream comprises at least one traffic frame;selecting a first back-off delay for said traffic frame from a firstback-off contention window; transmitting a traffic frame into ashared-communications channel at said first back-off delay; detecting acollision on said shared-communications channel during the transmissionof said traffic frame; selecting a second back-off delay for saidtraffic frame from a second back-off contention window; andre-transmitting said traffic frame into said shared-communicationschannel at said second back-off delay; wherein said first back-offcontention window is at least as long as said second back-off contentionwindow.
 2. The method of claim 1 wherein said second back-off contentionwindow is shorter than said first back-off contention window.
 3. Themethod of claim 1 further comprising: detecting a collision on saidshared-communications channel during the re-transmission of said trafficframe; selecting a third back-off delay for said traffic frame from athird back-off contention window; and again re-transmitting said trafficframe into said shared-communications channel at said third back-offdelay, wherein said second back-off contention window is at least aslong as said third back-off contention window.
 4. The method of claim 3wherein said third back-off contention window is shorter than saidsecond back-off contention window.
 5. An apparatus comprising: aprocessor for (i) generating a first traffic stream for transmissioninto a shared-communications channel, wherein said first traffic streamis the only privileged traffic stream contending for access to saidshared-communications channel, and wherein said first traffic streamcomprises at least one traffic frame, (ii) selecting a first back-offdelay for said traffic frame from a first back-off contention window,and (iii) selecting a second back-off delay for said traffic frame froma second back-off contention window; a transmitter for (i) transmittinga traffic frame into a shared-communications channel at said firstback-off delay, and (ii) re-transmitting said traffic frame into saidshared-communications channel at said second back-off delay; and areceiver for (i) detecting a collision on said shared-communicationschannel during the transmission of said traffic frame; wherein saidfirst back-off contention window is at least as long as said secondback-off contention window.
 6. The apparatus of claim 5 wherein saidsecond back-off contention window is shorter than said first back-offcontention window.
 7. The apparatus of claim 5 wherein: said receiver isfurther for (ii) detecting a collision on said shared-communicationschannel during the re-transmission of said traffic frame; said processoris further for (iv) selecting a third back-off delay for said trafficframe from a third back-off contention window; and said transmitter isfurther for (iii) again re-transmitting said traffic frame into saidshared-communications channel at said third back-off delay, wherein saidthird back-off contention window is shorter than said second back-offcontention window.
 8. The apparatus of claim 7 wherein said secondback-off contention window is at least as long as said third back-offcontention window.
 9. The apparatus of claim 7 wherein said thirdback-off contention window is shorter than said second back-offcontention window.
 10. A method comprising: generating a first trafficstream for transmission into a shared-communications channel, whereinsaid first traffic stream is the only privileged traffic streamcontending for access to said shared-communications channel, and whereinsaid first traffic stream comprises a first traffic frame; generating asecond traffic stream for transmission into said shared-communicationschannel, wherein said second traffic stream is not privileged, andwherein said second traffic stream comprises a second frame; selecting afirst back-off delay for said first frame from a first back-offcontention window; selecting a second back-off delay for said secondframe from a second back-off contention window; transmitting said firstframe into said shared-communications channel at said first back-offdelay; transmitting said second frame into said shared-communicationschannel at said second back-off delay; detecting a collision on saidshared-communications channel during the transmission of said firstframe; detecting a collision on said shared-communications channelduring the transmission of said second frame; selecting a third back-offdelay for said first frame from a third back-off contention window;selecting a fourth back-off delay for said second frame from a fourthback-off contention window; re-transmitting said first frame into saidshared-communications channel at said third back-off delay; andre-transmitting said second frame into said shared-communicationschannel at said fourth back-off delay; wherein said first back-offcontention window is as long as said third back-off contention window;and wherein said fourth back-off contention window is longer than saidsecond back-off contention window.
 11. The method of claim 10 whereinsaid third back-off contention window is shorter than said firstback-off contention window.
 12. The method of claim 10 wherein saidthird back-off contention window is the same as said first back-offcontention window.
 13. The method of claim 10 wherein said firstback-off contention window is the same as said second back-offcontention window.
 14. The method of claim 10 wherein said firstback-off contention window is shorter than said second back-offcontention window.
 15. An apparatus comprising: a processor for (i)generating a first traffic stream for transmission into ashared-communications channel, wherein said first traffic stream is theonly privileged traffic stream contending for access to saidshared-communications channel, and wherein said first traffic streamcomprises a first traffic frame, (ii) generating a second traffic streamfor transmission into said shared-communications channel, wherein saidsecond traffic stream is privileged, and wherein said second trafficstream comprises a second frame, (iii) selecting a first back-off delayfor said first frame from a first back-off contention window, and (iv)selecting a second back-off delay for said second frame from a secondback-off contention window, (v) selecting a third back-off delay forsaid first frame from a third back-off contention window, (vi) selectinga fourth back-off delay for said second frame from a fourth back-offcontention window; a transmitter for (i) transmitting said first frameinto said shared-communications channel at said first back-off delay,(ii) transmitting said second frame into said shared-communicationschannel at said second back-off delay, (iii) re-transmitting said firstframe into said shared-communications channel at said third back-offdelay, and (iv) re-transmitting said second frame into saidshared-communications channel at said fourth back-off delay; and areceiver for (i) detecting a collision on said shared-communicationschannel during the transmission of said first frame, and (ii) detectinga collision on said shared-communications channel during thetransmission of said second frame; wherein said first back-offcontention window is as long as said third back-off contention window;and wherein said fourth back-off contention window is longer than saidsecond back-off contention window.
 16. The method of claim 15 whereinsaid third back-off contention window is shorter than said firstback-off contention window.
 17. The method of claim 15 wherein saidthird back-off contention window is the same as said first back-offcontention window.
 18. The method of claim 15 wherein said firstback-off contention window is the same as said second back-offcontention window.
 19. The method of claim 15 wherein said firstback-off contention window is shorter than said second back-offcontention window.